In 1950, Lord Killanin becomes the head of the Olympic Council of Ireland (OCI), and becomes his country’s representative in the IOC in 1952. He becomes senior vice-president in 1968, and succeeds Avery Brundage, becoming President-elect at the 73rd IOC Session (August 21–24) held in Munich prior to the 1972 Summer Olympics. He takes office soon after the Games.

Lord Killanin serves as Honorary Consul-General of Monaco in Ireland from 1961 to 1984 and as Chairman of the Race Committee for Galway Racecourse from 1970 to 1985. A keen horse racing enthusiast, he also serves as a steward of the Irish Turf Club on two occasions and on the National Hunt Steeplechase Committee. In his business life Lord Killanin is a director of many companies including Irish Shell, Ulster Bank, Beamish & Crawford and Chubb Ireland. He is a founder member of An Taisce (The National Trust for Ireland) and is chairman of the National Monuments Advisory Council until his death.

Lord Killanin dies at his home in Dublin on April 25, 1999 at the age of 84 and, following a bilingual funeral Mass at St. Enda’s Church in Spiddal, County Galway, he is buried in the family vault in the New Cemetery, Galway.